1998 Ford Ranger loud noise and vibration

I was driving on the freway and after about 10 minutes I heard a faint sound and felt vibration then within 5 minutes it got really loud under ther truck. The noise sounded like it was coming from the transmission or 4wd gearbox but I had it in 2wd. It aslo could be the whel bearings but I didnt fel anything in the steering whel. After my truck sat for 2 hours I drove it again and that time it took about 15 minutes to make noise but then it was constant no matter what speed. Thank You